1. Field of the Invention
The present invention relates to a continuously variable transmission that is equipped with variable diameter pulleys and endless flexible members engaged with the pulleys so as to provide continuously variable speed ratio between the pulleys by adjusting groove widths of the pulleys, and also relates to a thrust applying method of the continuously variable transmission.
2. Description of the Related Art
A continuously variable transmission of this kind is disclosed in Japanese patent laying-open publication No. Hei 11-236965. As shown in FIG. 4, this continuously variable transmission includes a primary pulley 101, a secondary pulley 102, and a metal belt 103 mating with the pulleys 101 and 102, where the pulleys 101 and 102 are controlled to change their groove widths by application of the resultant force consisting of a fluid pressure force and a spring force. This spring force decreases the maximum fluid pressure to be applied to the pressure chambers of the pulleys 101 and 102 at the highest speed ratio and the lowest speed ratio, resulting in the reduction of fluid-pump capacity.
The above known conventional continuously variable transmission, however, encounters a problem that the reduction amount of the fluid-pump capacity is limited to be small. The reason is as follows: the fluid pressure forces are produced by introducing pressurized fluid into primary and secondary fluid pressure chambers of primary and secondary thrust applying mechanisms 104 and 105. The fluid pressure forces act only in the same direction on the pulleys 101 and 102 so as to narrow their groove widths, which causes the thrust difference between the primary and second pulleys 101 and 102 to be determined regardless of the spring forces, only by the difference between fluid pressure forces applied to them. This means that one of the pulleys 101 and 102 requires high pressurized fluid to be supplied so as to obtain the highest and lowest speed ratios and prevent a slippage between the belt 103 and the pulleys 101 and 102, limiting reduction of the pump capacity.
Specifically, FIG. 5 shows relationships of characteristics of necessary pulley thrust, fluid pressure forces, and spring forces with relative to speed ratio from a “HIGH” speed ratio to a “LOW” speed ratio, where a characteristic line L1 shows thrust (FDS) necessary for the secondary pulley 102; a characteristic line L2 shows fluid pressure force (FpS) to be applied to the secondary pulley 102; a characteristic line L3 shows spring force (FsS) of a secondary spring 107 acting on secondary pulley 102; a characteristic line L4 shows thrust (FDP) necessary for the primary pulley 101; a characteristic line L5 shows fluid pressure force (FpP) to be applied to the primary pulley 101; and a characteristic line L6 shows spring force (FsP) of a primary spring 106 acting on the primary pulley 101.
The secondary-pulley thrust FDS is obtained by adding the secondary spring force FsS to the secondary fluid pressure force FpS, which gives the following equation.FDS=FpS+FsS 
The primary-pulley thrust FDP is obtained by adding the primary spring force FsP to the primary fluid pressure force FpP, which gives the following the equation.FDP=FpP+FsP 
The thrust difference ΔFD at the lowest (the maximum LOW) speed ratio is obtained by subtracting the secondary-pulley thrust FDS from the first primary-pulley thrust FDP, which yields the following equation.ΔFD=FDP−FDS=(FpP+FsP)−(FpS+FsS)
When the primary and secondary spring forces FsP and FsS are set to have the same strength, the above equation of the thrust difference ΔFD at the lowest speed ratio is expressed as the following equation.ΔFD=FpP−FpS 
Similarly, the thrust difference ΔFD at the highest (the maximum HIGH) speed ratio is obtained by subtracting the primary-pulley thrust FDP from the secondary-pulley thrust FDS, and when the primary and secondary spring forces FsP and FsS are set to have the same strength, the equation of the thrust difference ΔFD at the highest speed ratio is expressed by the following equation.ΔFD=FpS−FpP 
The spring forces FsP and FsS can not be set larger than the necessary thrust FDP and FDS,
The above equations shows that the thrust differences ΔFD at the lowest and highest speed ratios are produced by differences between the primary and secondary fluid pressure forces FpP and FpS. This results in the fact that when the pressure PL supplied to one of the pulleys to be kept at lower pressure is set to obtain proper belt-clamping force, the pressure PH supplied to the other one of the pulleys to be kept at the high pressure needs to be set to be larger than the pressure PL by the pressure corresponding to the thrust difference ΔFD.
In order to obtain the sufficient thrust difference both at the lowest and highest speed rates, the pressures to be supplied to the fluid pressure chambers need approximately four to five times higher pressure, which requires higher fluid-pump capacity, resulting in degradation of fuel consumption especially when climbing a long slope at the lowest speed ratio or when high-speed running at the highest speed ratio.
When the spring forces are set different from each other between the primary and secondary springs 106 and 107, the similar problem occurs.
It is, therefore, an object of the present invention to provide a continuously variable transmission which overcomes the foregoing drawbacks and can obtain sufficient thrust difference between a primary pulley and a secondary pulley at the lowest and/or highest speed ratio with lower fluid pressure applied to the pulley to decrease a necessary fluid-pump capacity and improve fuel consumption.
It is another object of the present invention to provide a thrust applying method for a continuously variable transmission which overcomes the foregoing drawbacks and can obtain sufficient thrust difference between a primary pulley and a secondary pulley at the lowest and/or highest speed ratio with lower fluid pressure applied to the pulley to decrease a necessary fluid-pump capacity and improve fuel consumption.